The Addition of MoO₃ or SiO₂ Nano-/Microfillers Thermally Stabilized and Mechanically Reinforce the PVDF-HFP/PVP Polymer Composite Thin Films

Abstract

The properties of thin polymer films are influenced by the size of the fillers, their morphology, the surface properties and their distribution/interaction in the polymer matrix. In this work, thin polymer composite films with MoO₃ or SiO₂ nano and micro fillers in PVDF-HFP/PVP polymer matrix were successfully fabricated using the solvent casting method. The effects of different types, sizes and morphologies of the inorganic fillers on the crystallization of the PVDF-HFP polymer were investigated, as well as the effects on the thermal and mechanical properties of the composites. Scanning electron microscopy, ATR-FTIR spectroscopy, differential scanning calorimetry, nanoindentation and uniaxial mechanical tests were used for characterization. The results showed that MoO₃ nanowires thermally stabilized the polymer matrix, induced crystallization of the PVDF-HFP polymer in all three polymorphs (α-, β-, γ-phase) and formed a geometrical network in the polymer matrix, resulting in the highest elastic moduli, hardness and Young’s modulus.

1. Introduction

Polymer composites are promising new engineering materials with improved thermal, mechanical, and surface properties. Various fillers have been used to improve different properties of polymers, i.e., carbon nanotubes [1], glass fibers [2], cellulose [3], silica-SiO₂ particles [4], SiC and Al₂O₃ nanowires [5], and others. The particle size and morphology of the fillers, their orientation, dispersion, and concentration in polymer matrix, the adhesion between the polymer matrix and the filler and the interphase properties have already been proved to have a major influence on the final properties of the composites. It has already been shown that the particle size has a great influence on the mechanical properties, such as Young’s modulus [6], which is due to the different degree of interactions between the filler and the polymer matrix. For spherical Al₂O₃ particles, it was found that the addition of microparticles has only a small effect on the Young’s modulus, while the Young’s modulus increases significantly with the addition of nanoparticles [7].

The polymer composites based on polyvinylidene fluoride (PVDF) or poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymers are used in various attractive applications, i.e., as flexible, biocompatible nanofibers with superior piezoelectric/ferroelectric properties in the field of bone tissue regeneration [8], flexible electronic skins [9], nanogenerators [10], or intelligent and smart packaging material with water vapor impermeability [11]. Additionally, magnetoactive polymer composites with CoFe₂O₄ nanoparticles in a polyvinyl alcohol (PVA) polymer matrix are being investigated for applications in the fields of remote sensing, soft robotics, electronics, and biomedicine [12]. Another useful application is flexible energy storage devices such as supercapacitors for extreme conditions [13]. For the prediction of electric field distribution in the PVDF based nanocomposites with various fillers, the researchers in the last years have used COMSOL software [14].

The increase in crystallinity affects not only the morphological characteristics but also the mechanical and thermal properties [15]. Factors such as particle size and particle-particle/particle-matrix interactions influence the crystallization of polymers with crystalline characters. The PVDF-HFP polymer is a semi-crystalline polymer that can crystallize into α-, β-, and γ- phases, depending on the temperature, processing conditions and fillers/additives used. The doping of PVDF-HFP polymer with fillers, most commonly organic particles [16] or clay minerals [17], induce the crystallization in the most polar β-phase, which was also formed in the case of mechanical deformation of non-polar α-phase [18]. Especially, the addition of water-soluble polyvinylpyrrolidone (PVP) polymer induces the crystallization of PVDF-HFP polymer in β-phase, and even more, PVP increases its stabilization [19].

During the evaporation process of the solvent (solidification) the molecular chains adjacent to the particles contract earlier than the free polymer chains, forming a high-density zone with higher elastic modulus around the particles [20]. The polymer matrix as the main component of the polymer composite carries the mechanical load, while the dispersed particles primarily increase the material’s elastic modulus [21]. Moreover, the incorporation of nanoparticles into the polymer matrix significantly improves the structure of the cross-linked network between the polymer matrix and the filler, as they serve as cross-linking attractors, which is particularly observed in the case of large deformations [22].

For the formation of high-strength composites, the adhesion between the filler and the polymer matrix is a very important parameter that depends on the geometry of the filler, the chemical structure of the polymer, the regularity of the molecular structure, the conformational properties, and the branching of the polymer chains [21]. Furthermore, Woigk et al. reported that interfacial properties correlate with strength in both longitudinal and transverse directions in a nanocomposite based on a polylactic acid (PLLA) polymer reinforced with natural fibers [23].

MoO₃ nanowires are used in a diverse range of applications, i.e., they can enhance the antimicrobial activity of polymer nanocomposite films [19,24,25]. However, the fabrication of these nanoparticles is limited due to its complex nature and high cost. On the other hand, SiO₂ filler, which has not yet been used in the PVDF-HFP/PVP polymer matrix, is widely used in the pharmaceutical industry. This nanomaterial could be a promising candidate to induce antifouling and antimicrobial properties in the polymer matrix. In our previous study, we showed that the SiO₂ nanoparticles in PVDF-HFP/PVP based polymer nanocomposite reduced the adhesion of S. aureus and E. coli bacterial cells after 24 h of incubation [26]. Since SiO₂ filler is more affordable, its use would improve the availability and application possibilities of such polymer composites.

In this research, we investigated the effects of adding morphologically different inorganic fillers, such as MoO₃ and SiO₂ on the thermal, structural and mechanical properties of polymer composites based on PVDF-HFP and PVP polymer. Furthermore, we compared how different filler sizes affect crystallization, the presence of the individual crystal phases and the interactions between fillers and the polymer matrix, which also determines the mechanical properties of polymer composites.

2. Materials and Methods

The polymer composites were prepared using poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) (Sigma Aldrich, St. Louis, MO, USA) and polyvinylpyrrolidone (PVP) polymers (Sigma Aldrich, St. Louis, MO, USA) polymer. MoO₃ nanowires (Nanotul Ltd, Ljubljana, Slovenia), MoO₃ microparticles (Sigma Aldrich, St. Louis, MO, USA), SiO₂ nanoparticles (W.R. Grace, Columbia, MD, USA), and SiO₂ microparticles (W.R. Grace, Columbia, MD, USA) were used as fillers. For each polymer composite, both polymers were dissolved separately in dimethylformamide (DMF, Carlo Erba Reagents, Cornaredo, Italy) and mixed for 4 h at 80 °C and 300 rpm on a magnetic stirrer. The fillers were then added to the PVP polymer solution, and the dispersion was mixed for further 2 h under the same conditions. In the last step of the mixing process, the PVDF-HFP polymer solution was added to the dispersion and mixed for 2 additional hours. After the mixing process, the polymer composite solutions were casted on a Teflon plate and dried at 80 °C for 2 h. The composition of the polymer composites was constant, with the mass ratio of PVDF-HFP, PVP and filler being 69 wt. %, 23 wt. %, and 8 wt. %, respectively. The prepared polymer composites are shown in Figure 1.

The surface topography of polymer composite thin films was investigated with a scanning electron microscope Verios 4G HP, Thermo Fisher (Waltham, MA, USA). The images were generated from the signal of secondary electrons at a working distance of about 4 mm and an accelerated voltage of 3–5 kV. The nano- and micro-particles in powder form were placed on an adhesive carbon tape fixed in the sample holder and blown with argon to remove the excess powder. The polymer composite thin films were placed on an adhesive carbon tape and sputtered with a 10 nm carbon layer.

The ATR-FTIR spectra were recorded at room temperature with a FTIR spectrometer (Perkin Elmer, Spectrum 65, Waltham, MA, USA), equipped with a single reflection diamond crystal. The ATR-FTIR spectra of free-standing polymer composite thin films were recorded in a range from 4000 to 600 cm⁻¹, with 64 scans per spectrum and a resolution of 4 cm⁻¹.

Differential scanning calorimetry-DSC was performed using the Q2500 calorimeter (TA Instruments, New Castle, DE, USA). Heat–cool–reheat tests were performed in a nitrogen atmosphere in the temperature range from −90 °C to 180 °C with heating/cooling rates of 10 °C/min. The thermal properties of the polymer composites, i.e., the melting and crystallization temperature, the glass transition temperature and the enthalpies were evaluated using the TRIOS software. From these data, the degree of crystallinity of the polymer composites was calculated using the following equation:

$${\chi }_c={\displaystyle \frac{∆H_f}{∆H_f^0·f}}·100\%$$

where Χ_c is the degree of crystallinity, $∆H_f$ is the fusion enthalpy, $∆H_f^0$ is the fusion enthalpy for 100% crystallinity (for PVDF-HFP polymer, the value of 104.7 J/g was used [27]) and f is the weight fraction of PVDF-HFP polymer in composite.

The elastic modulus and hardness on the surface of polymer composite films were determined using nanoindentation. The free-standing polymer composite films were placed on the glass coated with two-component glue and fixed on a holder. The continuous stiffness measurements (CSM) were performed by Nanoindenter G200 XP instrument (Agilent Technologies, Santa Clara, CA, USA) using a standard three-sided pyramidal Berkovich probe at the tip oscillation frequency of 45 Hz and 2 nm harmonic amplitude. For each sample, the 36 indents were performed. The values of elastic modulus and hardness were determined at a depth of 1200 and 2400 nm and Poisson’s ratio of 0.4 for PVDF-HFP [28].

Uniaxial mechanical tests were carried out by the modular compact rheometer MCR 702 (Anton Paar, Graz, Austria) with samples, which were 5 mm wide, 40 mm long and 150 µm thick. The measurements were carried out at room temperature with pre-applied force of 0.3 N in three repetitions for each polymer composite film. Average stress-strain curves were presented as the result of the characterization. Young’s modulus was calculated as a slope of stress-strain curves in a linear range.

3. Results and Discussion

3.1. Morphology

The properties of polymer composites depend on the morphology, size and distribution of the filler within the polymer matrix. Therefore, the MoO₃ and SiO₂ fillers, which differ in size and morphology, were used to produce the polymer composites. The morphology of all four polymer composite fillers is shown in Figure 2. The MoO₃ nanowires (with a diameter of about 200 nm and a length of up to 3 μm), which grow in an ortho-rhombic crystal structure, have a large specific surface area (12.06 ± 0.05 m²/g) [29] (Figure 2a). The commercially available MoO₃ microparticles also exhibit an orthorhombic crystal structure, with an average single crystal size of about 10 μm [29]. Since they show a tendency to form larger agglomerates (Figure 2b), they have smaller specific surface area of 0.27 ± 0.05 m²/g [29]. The mesoporous SiO₂ nanoparticles, which tend to form agglomerates (Figure 2c), have the largest specific surface area of (345.55 ± 1.26 m²/g) [30]. In contrast, the SiO₂ microparticles were the largest filler particles with irregular shapes and an average particle size of about 150 µm (Figure 2d). They have a specific surface area of 305.51 ± 1.23 m²/g [31].

The surface topography of PVDF-HFP/PVP polymer matrix and polymer composite films with MoO₃ and SiO₂ nano- and micro-particles is shown in Figure 3. Partially circular polymer domains can be observed on the surface of the polymer matrix (Figure 3a) and the polymer composite filled with MoO₃ particles (Figure 3b,c). The viscoelastic properties of PVDF-HFP and PVP polymer solution were different, which leads to the formation of phase separation structures in polymer composites during the film formation process. The PVP domains with fillers in the PVDF-HFP polymer matrix are formatted, which appear as spherical structures on the surface. The incorporation of PVP reduces the overall crystallinity of PVDF-HFP, which can promote the formation of a more amorphous and nanostructured surface. The interaction between PVP and PVDF-HFP induces the self-assembly of PVP into spherical nanostructures with an average diameter of 200 to 500 nm [19]. However, when MoO₃ nano- or microparticles were added, these spherical structures increase in size to ~2 µm. The MoO₃ nanowires were homogeneously dispersed on the surface of the polymer composite (Figure 3b). In addition, the polymer domains were also visible on the surface of the polymer composite with MoO₃ microparticles (Figure 3c), where the rough surface was observed because of the larger dimension of the fillers, which were, however, relatively homogeneously dispersed. The coatings with the SiO₂ nanoparticles consist of agglomerated particles (Figure 3d), which are visible on the surface due to the very low density of the nanoparticles which prevent their sedimentation in the polymer matrix. In contrast, when SiO₂ microparticles were added, the particles did not agglomerate, but larger microparticles are visible on the surface (Figure 3e).

3.2. Structural Properties

The structure of polymer composites, in particular the interactions between MoO₃ or SiO₂ nano/micro fillers and the polymer matrix, was investigated by ATR-FTIR spectroscopy. The vibrational modes of the chemical bonds provide information about the crystalline phases in polymer composites and the dispersity of the fillers in the polymer matrix, which is reflected in a shift of peak intensity and position.

The ATR-FTIR spectra of PVDF-HFP/PVP polymer matrix and polymer composites with MoO₃ and SiO₂ nano/micro fillers are shown in Figure 4. FTIR spectroscopy shows the presence of mixed crystal phases in the polymer composites, however the ratio between the phases was different depending on the type of filler.

In the PVDF-HFP/PVP polymer matrix film, the peaks corresponding to the electroactive phases (β + γ) of PVDF-HFP were observed at 840 cm⁻¹. Additionally, the combination of the α + β + γ phases were observed as a collective peak at 1176 cm⁻¹ (symmetric stretching of –C–C–C-bond [32]) and 1401 cm⁻¹ in FTIR spectra and at 880 cm⁻¹ and 1403 cm⁻¹ in Raman spectra [33]. These high intensity peaks (at 840 cm⁻¹, 880 cm⁻¹ and 1403 cm⁻¹) correspond to rocking of CH₂ and asymmetric stretching of CF₂ groups, symmetric stretching of CF₂ and C-C groups [34], and CH₂ swinging [35]. The clearly visible peak at 1234 cm⁻¹ also in the PVDF-HFP/PVP polymer matrix film is the exclusive peak, typically used for the identification of γ phase [36]. A strong peak at about 1668 cm⁻¹ was attributed to C=O and C-N stretching in the PVP polymer [37].

The bands observed at 1423 cm⁻¹, 1462 cm⁻¹ and 1494 cm⁻¹ correspond to the bending of the CH₂ group in the PVP polymer [34,37] which was present in all spectra. The peak at 1072 cm⁻¹ is a composition of two overlapping peaks of PVDF and SiO₂. In the case of the addition of SiO₂ filler, the peak at 1060–1070 cm⁻¹ [38] is characteristic of the Si-O bond and has caused the broader peak. However, this peak was present also in all spectra of nanocomposite, which is due to the out-of-plane vibration of CF₃ bound in the PVDF-HFP polymer [39]. Furthermore, the shift to a higher wavenumber of the peak at 1184 cm⁻¹ was observed in the samples with SiO₂ addition.

The polymorphic crystalline polymer PVDF-HFP can exhibit different mechanical properties depending on the presence of a crystal phase in the composite material. The characteristic peaks for the γ-phase of the PVDF-HFP polymer at 811 cm⁻¹, 1234 cm⁻¹, and 1429 cm⁻¹ are present in spectra with MoO₃/SiO₂ microparticles and SiO₂ nanoparticles. However, in the composites with SiO₂ microparticles and MoO₃ nanoparticles, the absorption peak at 614 cm⁻¹ indicates the formation of a small amount of α-phase. The increase in α-phase was observed in the composite with MoO₃ nanowires, while characteristic peaks at 1209 cm⁻¹ and 1423 cm⁻¹ were found in the spectra. Moreover, the highest intensity of the peaks at 1276 cm⁻¹ (typically an exclusive peak for β phase) and 840 cm⁻¹ also indicates the presence of a larger amount of β-phase in the composite with MoO₃ nanowires. Meanwhile, the peak corresponding to the CF₂ symmetrical stretching at 1424 cm⁻¹ [40] has the highest intensity in the case of MoO₃ nanowires addition. According to these results, we can expect the best mechanical properties of polymer nanocomposite with MoO₃.

The SiO₂ nano/microparticles induced the crystallization of the PVDF-HFP polymer in the γ-phase, while SiO₂ microparticles promote the formation of the α-phase. The γ-phase is usually formed by the addition of the fillers such as clays and by solid phase transformation from the non-polar α-phase to the polar γ-phase [41]. In addition, the low evaporation rate of DMF in the oven (at 80 °C) reduces the mobility of the polymer chains, favoring the formation of the β-phase [42]. Moreover, nucleation in the γ-phase is also a consequence of the rapid evaporation of the solvent [42]. Furthermore, the mesoporous SiO₂ nanorods induced the formation of the polar β-phase of PVDF due to the intermolecular hydrogen interactions in the interphase [43], as observed in the characteristic FTIR peak at 1276 cm⁻¹. On the other hand, not only the filler but also the PVP polymer has an influence on the crystallization of PVDF; PVP also stabilized the β-phase of PVDF [19].

3.3. Thermal Properties

The DSC curves of the polymer matrix and polymer composites with MoO₃ and SiO₂ nano/micro fillers are shown in Figure 5, where Figure 5a shows the reheating curves and Figure 5b the cooling curves. The values of the glass transition temperatures (T_g), melting temperatures (T_m) and enthalpy of fusion (ΔH) were determined from the reheating curves, while the crystallization temperatures (T_c) and enthalpies were determined from the cooling curves. All temperatures and enthalpies are listed in

Table 1. Parameters of DSC curves for pure PVDF-HFP, polymer matrix (PVDF-HFP/PVP) and polymer composites with MoO₃ and SiO₂ nano- and micro-particles.

Polymer Films with	Tg [°C]	Tm [°C]	ΔHm [J/g]	Tc [°C]	ΔHc [J/g]	XC [%]
PVDF-HFP	−35.66	142.00	36.42	110.39	32.60	34.79
PVDF-HFP/PVP	−40.33	130.19	30.78	98.49	10.09	40.01
MoO3 nanowires	−38.42	127.50	8.87	29.60	2.31	12.28
MoO3 microparticles	−30.96	124.41	4.31	22.53	0.42	5.96
SiO2 nanoparticles	−32.82	124.36	9.19	15.46	0.93	12.72
SiO2 microparticles	−32.95	128.07	7.94	20.96	0.73	11.00

. The lowest glass transition temperature was determined for PVDF-HFP/PVP polymer matrix, which was 6.5 °C lower compared to the pure PVDF-HFP film [18]. When fillers were added to the polymer matrix, the polymer composite with MoO₃ nanowires exhibited the lowest glass transition temperature (−38.42 °C), while the phase transitions were similar for the other composites. The melting temperature of PVDF-HFP/PVP polymer blend was ~12 °C lower compared to the pure PVDF-HFP film (T_m = 142.00 °C). This temperature shift is due to the addition of PVP polymer, as it acts as a plasticizer and can absorb moisture at room conditions [18]. The addition of MoO₃ microparticles and SiO₂ nanoparticles also lowered the melting temperature of the composites by about 2–6 °C compared to the PVDF-HFP/PVP polymer matrix. The highest melting enthalpy was observed with the addition of SiO₂ nanoparticles, followed by MoO₃ nanowires and SiO₂ microparticles, while the lowest value was observed for the polymer composite with MoO₃ microparticles. Moreover, the crystallinity of the polymer composites was reduced in the same order from 12.72% to 12.28%, 11.00% and 5.96%, respectively.

3.4. Mechanical Properties

The mechanical properties of the polymer composite surfaces were evaluated by nanoindentation. The elastic modulus (Figure 6a) and hardness (Figure 6b) are shown because of the measurements. Compared to the pure polymer blend PVDF-HFP/PVP, the addition of MoO₃ microparticles and SiO₂ fillers did not significantly change the elastic modulus and hardness. The elastic modulus of the polymer composite with MoO₃ microparticles was 1.85 ± 0.02 GPa, with SiO₂ nanoparticles 1.73 ± 0.01 GPa, and with SiO₂ microparticles 1.86 ± 0.02 GPa. The highest hardness (0.16 ± 0.001 GPa) of the polymer composite surface was observed in the case of the addition of MoO₃ nanowires, which was due to the highest stiffness of the polymer chains fixed in the nanowire network. However, the other fillers slightly increased the hardness of the pure polymer matrix surface (0.06 ± 0.001 GPa [18]), with 0.076 ± 0.001 GPa for MoO₃ microparticles, 0.072 ± 0.0005 GPa for SiO₂ nanoparticles and 0.088 ± 0.001 GPa for SiO₂ microparticles, respectively.

In addition, Figure 7a shows the stress-strain curves of the polymer composites examined. The test results indicate that the fillers in nano-dimensions significantly enhance the mechanical properties with MoO₃ nanowires achieving the highest extensional stress values at the same strain, followed by SiO₂ nanoparticles. However, when microparticles were added, the mechanical behavior of the polymer composite was almost independent of the particle type. The same trend was observed for the Young’s modulus (Figure 7b), which describes the mechanical properties of the material and indicates stiffness and resistance to deformation by external forces. The highest stiffness of the polymer composite was achieved with the addition of MoO₃ nanowires, attributed to the largest number of internal connections or bonds between the filler and the polymer chains. The Young’s modulus value for the PVDF-HFP/PVP polymer matrix was 1347.6 ± 8.3 MPa, while it was lower for the polymer composite with MoO₃ nanowires (1544.1 ± 8.8 MPa), but higher when SiO₂ nanoparticles were added (1199.7 ± 7.6 MPa). This could be due to the high degree of crystallinity of the PVDF-HFP/PVP polymer matrix, which makes the polymer film stiffer. The polymer composites with the addition of MoO₃ or SiO₂ microparticles exhibited comparable Young’s modulus values (882.5 ± 6.9 MPa for MoO₃ and 893.3 ± 3.5 MPa for SiO₂).

While the addition of fillers generally improves thermal and mechanical properties, the combined effects of the matrix type, nanoparticle size, and dispersion quality are also very important [42]. Due to synergy of load transfer from the matrix to reinforcement particles in nano-dimensions the nanocomposite with MoO₃ nanowires exhibited the best mechanical properties. Moreover, the Young’s modulus was improved for the polymer nanocomposite with MoO₃ nanowires, followed by the mesoporous SiO₂ nanoparticles with an average pore size of 2.6 nm and a total pore volume of 0.17 cm³/g [44]. The Young’s modulus was similar for the composites with the microparticles, where the interactions between the particles and the polymer matrix were almost independent of the particle size and the type of inorganic filler.

The Young’s modulus for PVDF-HFP is 1560 MPa which decreases to 330 MPa with increasing addition of the MgCl₂·6H₂O filler (4 wt. %) [45]. On the other hand, polymer blends consisting of a PVDF/PVP showed a much higher value of Young’s modulus (~2930 MPa), which was further improved to ~2960 MPa by the addition of 7 wt.% of lanthanated CoFe₂O₄ nanoparticles (CLFO) [46].

The particle size in polymer matrix influences the mechanical behavior of composite due to the different degree of interactions between particles and the host polymer matrix or, in other words, different molecular-chain-network interactions [20]. The particles act as junctions among molecular chains of polymers and additionally improve the interactions in polymer matrix [20].

4. Conclusions

In this study, the influence of incorporating two different types and sizes of particles into a PVDF-HFP/PVP polymer matrix using the solvent casting method to produce thin polymer composite films was investigated. Furthermore, SiO₂ nanoparticles were used as a comparison and possible substitute to the currently-used MoO₃. SiO₂ nanoparticles are widely used, inexpensive and widely available, while the production of MoO₃ nanowires is very limited. Both MoO₃ and SiO₂ particles are promising materials for environmental applications, such as antimicrobial or antifouling coatings.

The presented study showed that the addition of nano/micro MoO₃ or SiO₂ fillers to the polymer matrix, which consists of PVDF-HFP and PVP polymer, improved the thermal and mechanical properties of the polymer composites. In addition, the MoO₃ nanowires formed a geometric network in the polymer matrix, resulting in the best mechanical properties, i.e., the highest elastic modulus, hardness and Young’s modulus. This improvement is consistent with the identification of the ferroelectric polar β-crystal phase of the PVDF-HFP polymer in this nanocomposite. On the other hand, MoO₃ microparticles predominantly induced the γ-phase. The SiO₂ particles induced the crystallization of the PVDF-HFP polymer in the γ-phase, but a small amount of α-phase was also identified in the composite with the addition of SiO₂ nanoparticles. However, metal oxide particles in the form of microsized contributed to mechanical properties less effectively due to weaker polymer matrix-filler interactions. The results emphasize the importance of particle size and its distribution in achieving the desired coatings properties.